Estimating rear spring weight.

[donraye]

Hi all, I bought a ride height adjustable coil-over conversion kit for my car and it come with rear springs of 150 lb./in. weight. I am about to put the kit in and am wondering if that spring weight is about right for the car or if I need to go lighter(maybe 125 or even 100lbs). The car is a Street Beast Cobra with a stock Mustang 8.8" 4 link and I am guessing the total car weight to be about 2,500+ lbs with gas. With me and the wife would be anonther 345 or so. Do you count the gear and passengers when you figure this or not? Do you simply take half of the total wet weight of the car to figure the rear weight or is that all wrong? Can anyone give me an idea of what spring weight would be a good place to start or how to estimate this? The car will likely not be used for competition driving, just street use. I would hate to put these 150 lb. springs in and then have to change them out right after. I am not the worlds best mechanic to say the least and it will probably be challenge enough for me just to do this once, let alone a second time right away. I have no way to actually weigh the car at the rear so what is the way to make a good guess? Any help would be appreciated!
Thanks, Don

[xlr8or]

Not a Street Beast but my car has the independant rear and runs 600# springs with 450# springs in the front

[donraye]

Originally Posted by xlr8or

Not a Street Beast but my car has the independant rear and runs 600# springs with 450# springs in the front

Not really the same thing as independents have a "leveraged" suspension and require much higher weights than solid rears. But thanks for the reply.
Don

[cdnus]

I have a 8.8 with the coilovers mounted outboard of the frame---about as far out as I could get them & still run a 295 tire. Started with 250# & they were too soft, I am at
275's now & they seem to be perfect for my use. Steet only with some spirited 0-60's.
If you are mounting them in the original location on the stock lower control arms I have no idea where you would start.

Craig

[Tim Brewer]

Are you sure you have 150# springs? You'll need at least 250#'s and probably heavier. With 150's you'll bottom on every dip in the road. Don't rack your brain trying figure and factor everything. It's better to be a little stiff, than too soft. I just ordered 300's for my car.

[donraye]

Originally Posted by Tim Brewer

Are you sure you have 150# springs? You'll need at least 250#'s and probably heavier. With 150's you'll bottom on every dip in the road. Don't rack your brain trying figure and factor everything. It's better to be a little stiff, than too soft. I just ordered 300's for my car.

I'm quoting the spring weight of EACH spring, so the pair is 300 lbs. Sorry if that seems confusing somehow.
Don

[mickmate]

These cars should be close to 48/52 Front Rear weight distribution. The 42 gallon tanks in the back can make a big difference but you have a Mustang tank I believe.

[donraye]

Originally Posted by mickmate

These cars should be close to 48/52 Front Rear weight distribution. The 42 gallon tanks in the back can make a big difference but you have a Mustang tank I believe.

No Mustang tank, no Mustang II front rack, etc. I have the big aluminum type tank like most everyone else unless they are running plastic/molded cells. This car does not have any "doner" parts except for the beefed up rebuilt 8.8" Mustang posi rear. All else is new, frame up. Thanks for the post.
Don

[ratsnst1]

Hey, Tim, I like your coler choice, yours is the first red and silver cobra I have seen. looks very nice, I have 250 pounders in mine, I might need to go to 300, but I am very close.My cobra is red as well Tim, I dont see very many of them.

[Tim Brewer]

Thanks Rat, your car looks real sharp too. I really like your polished wheels,sets your whole car off. I guess the "rat" means your engine is a BBC?

[Tim Brewer]

That's what I'm saying.. it IS 300 each that you want. When anyone is talking of spring rate, they are talking about the rate of one spring only, never of one multiplied by two.

[FST HSKR]

Don,

I am running 300 on the back of mine. It will ride a little rough. Took a lot of the push out of the car by raising the front end, then adjusting each of the ones on the rear. It won't take long and you will feel the difference not only front to back but side to side. Good luck.

Jim

[donraye]

Originally Posted by FST HSKR

Don,

I am running 300 on the back of mine. It will ride a little rough. Took a lot of the push out of the car by raising the front end, then adjusting each of the ones on the rear. It won't take long and you will feel the difference not only front to back but side to side. Good luck.

Jim

Originally Posted by Tim Brewer

That's what I'm saying.. it IS 300 each that you want. When anyone is talking of spring rate, they are talking about the rate of one spring only, never of one multiplied by two.

Originally Posted by Tim Brewer

Are you sure you have 150# springs? You'll need at least 250#'s and probably heavier. With 150's you'll bottom on every dip in the road. Don't rack your brain trying figure and factor everything. It's better to be a little stiff, than too soft. I just ordered 300's for my car.

Originally Posted by cdnus

I have a 8.8 with the coilovers mounted outboard of the frame---about as far out as I could get them & still run a 295 tire. Started with 250# & they were too soft, I am at
275's now & they seem to be perfect for my use. Steet only with some spirited 0-60's.
If you are mounting them in the original location on the stock lower control arms I have no idea where you would start.

Craig

Now I am totally confused. I got my spring weights from this spring weight application chart on the "Competition Engineering" web site
No. Description/Application
C2550 Spring Rate: 85 lbs/in., Total Weight on Rear Wheels: under 1050 lbs.
C2555 Spring Rate: 100 lbs/in., Total Weight on Rear Wheels: 1050 -1150 lbs.
C2560 Spring Rate: 125 lbs/in., Total Weight on Rear Wheels: 1150 - 1250 lbs.
C2565 Spring Rate: 150 lbs/in., Total Weight on Rear Wheels: 1250 - 1450 lbs.
C2570 Spring Rate: 200 lbs/in., Total Weight on Rear Wheels: 1450 - 1800 lbs.
Are these weights all wrong or ?
Are you guys saying I need to double these spring weights to be in the correct range. Which would mean I probably need a pair of 250-300 lb springs?
Please help clarify this if you would.
Thanks, Don

[FST HSKR]

Don,
The spring rating is for one side, so you would need to get 2 250 springs or 2 300 springs for the rear. In most race applications it is possible to have a different spring rate on each corner of the car.

Hope this helps,

Jim

[donraye]

Hi Jim, Yes I now understand that the weights you mentioned are for each side. Just having some trouble trying to understand why the chart I used (above) is so completely off. According to that chart I would be using 150 lbs./in. springs.
Perhaps that chart is for stock application on Mustangs which have a lot more wheel travel and much more room in the wheell wells than a Cobra kit.
I just ordered a pair of 275 lb. springs and will likely use those as my starting point and sell the 150s to someone on eBay or?
Thanks, Don

[ratsnst1]

Speaking of a heavier spring rate, does anyone know if I can run 3OO POUND SPRNG INSTEAD OF A 250 ON THE RIGHT REAR SIDE ONLY, THATS THE SIDE i SEEM TO RUB AT UNDER A HEAVY LOAD, OR ANOTHER WORD WOULD BE MASHING MY FOOT INTO IT .